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Convergent Boundaries
A convergent boundary is a boundary between two plates that are moving
toward each other, or converging. Two broad classifications for convergent
boundaries are subduction boundaries and collision boundaries.
Subduction Boundaries
When an oceanic plate plunges beneath another plate, the oceanic plate
is said to be subducting (suhb-DUHK-tihng) beneath the overriding plate.
The boundary between the plates is called a subduction boundary.
One important feature of a subduction boundary is a long, deep trench
called a deep-sea trench that forms along the boundary. Such trenches
are the deepest parts of the ocean floor.
Subduction boundaries can occur at the convergence of two oceanic
plates or at the convergence of an oceanic plate with a continental plate.
When two oceanic plates converge, the deep-sea trench that forms is
accompanied by the formation of a chain of volcanic islands called a
volcanic island arc on the overriding plate. For example, as the Pacific Plate
subducts under the Philippine Plate, the Pacific Plate is pulled down to
form the Mariana Trench. The leading edge of the overriding Philippine
Plate is marked by a chain of volcanic islands, the Mariana Islands.
When an oceanic plate converges with a continental plate, the denser
oceanic plate subducts beneath the less-dense continental plate.
A deep-sea trench forms, as shown below. A mountain chain and volcanoes
form inland on the overriding continental plate.
DIVERGENT BOUNDARY
These hydrothermal vents
are on the Mid-Atlantic
Ridge.
The prefix sub- in the words
subducting and subduction means
“below.” Subduction comes from a
Latin verb meaning “to draw away
from below.”
Image not available.
Please refer to the
image in the textbook
or in the eEdition CD.
Mid-ocean ridge
Rift valley
Deep-sea trench
SUBDUCTION
BOUNDARY
Mount Shasta
is a volcano in
California that
has formed near
a subduction
boundary.
Mountain chain
with volcanoes
Convergent boundary
Divergent
boundary
ocean-continent
subduction
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